The present invention relates to a flasher type fish sonar in which two ultrasonic waves having wide and narrow directive angles are transmitted in water and the echo signals of the two ultrasonic waves reflected in the water are received and flashed in the same portions of a flasher display by different colors.
Generally, in the fish sonar, when an ultrasonic transducer is put in the water and an ultrasonic pulse wave is transmitted from the ultrasonic transducer to the water, the ultrasonic pulse wave is propagated to the bottom of the water by conically spreading in an angle (a directive angle), one part of the echo ultrasonic pulse wave reflected by fishes and the residual echo ultrasonic pulse wave reflected by the bottom in the water are returned to the ultrasonic transducer and are converted to the electric signal in the ultrasonic transducer. Then, the time difference between the transmitting of the ultrasonic pulse wave and the receiving of the echo ultrasonic pulse wave is displayed as a distance information by a display means and we can see the fishes and the bottom in the water.
In such a fish sonar, when the transmitted ultrasonic pulse wave has one frequency (one directive angle), the distance between the water surface and the fishes in the directive angle of the ultrasonic pulse wave is detected by the position relation between the surface and the bottom in the water. However, the bottom and the fishes in the water, which are removed from the directive angle of the ultrasonic pulse wave, cannot be detected.
Also, there is proposed a fish sonar in which the two ultrasonic pulse waves having different frequencies and the wide and narrow directive angles are transmitted to the water by the ultrasonic transducers and the echo ultrasonic waves are received by the ultrasonic transducers and are displayed in the separate display portions of the display means. However, the fish positions must be decided by comparing the separated pictures with each other.